6 On Some Interactions of Organisms. 



a species can not long maintain itself in numbers greater 

 than can find sufficient food, year after year. If it is a phy- 

 tophagous insect, for example, it will soon dwindle if it 

 seriously lessens the numbers of the plants upon which it 

 feeds, either directly, by eating them up, or indirectly, by 

 so weakening them that they labor under a marked disad- 

 vantage in the struggle with other plants for foothold, 

 light, air, and food. The interest of the insect is therefore 

 identical with the interest of the plant it feeds upon. 

 Whatever injuriously affects the latter, equally injures the 

 former ; and whatever favors the latter, equally favors the 

 former. This must, therefore, be regarded as the extreme 

 normal limit of the numbers of a phytophagous species, — 

 a limit such that its depredations shall do no especial harm 

 to the plants upon which it depends for food, but shall re- 

 move only the excess of foliage or fruit, or else superflu- 

 ous individuals which must either perish otherwise, if not 

 eaten, or, surviving, must injure their species by over- 

 crowding. If the plant-feeder multiply beyond the above 

 limit, evidently the diminution of its food supply will 

 soon react to diminish its own numbers ; a counter reaction 

 will then take place in favor of the plant, and so on through 

 an oscillation of indefinite continuance. 



On the other hand, the reduction of the phytophagous 

 insect below the normal number will evidently injure the 

 food plant by preventing a reduction of its excess of growth 

 or numbers, and will also set up an oscillation like the 

 preceding, except that the steps will be taken in reverse 

 order.* 



I next point out the fact that precisely the same reason- 

 ing applies to predaceous and parasitic insects. Their in- 

 terests, also, are identical with the interests of the species 

 they parasitize or prey upon. A diminution of their food 

 reacts to decrease their own numbers. They are thus vi- 

 tally interested in confining their depredations to the ex- 

 cess of individuals produced, or to redundant or otherwise 

 unessential structures. It is only by a sort of unlucky acci- 

 dent that a destructive species really injures the species 

 preyed upon. 



*See "Principles of Biology," by Herbert Spencer, Vol. II., pp. 397-478. 



